(1) Field of the Invention
This invention relates to apparatus for melting metal scrap material, particularly aluminum scrap material. More particularly, the invention relates to apparatus for pre-treating metal scrap material and for transferring the pre-treated scrap material to a melting furnace, e.g. a reverberatory furnace or direct-charged convective furnace.
(2) Description of the Related Art
Scrap materials made of aluminum or other metals (e.g. copper) can be re-melted for re-use, but they may be contaminated and may require pre-treatment to remove or reduce such contaminants before they can be added directly to a melting furnace.
For example, reverberatory furnaces are widely used to re-melt aluminum scrap material for eventual re-use by metal fabricators, although they are also used for melting pure metals. Direct-charged convective furnaces are normally used for melting metals having lower melting temperatures, such as babbit metal or lead. Scrap metal for re-melting may be in the form, for example, of sows (large castings), ingots (e.g. T-bar ingots) and other metal pieces (often referred to as “small form”). However, metal foil is so thin that it would be converted almost entirely to oxide in such furnaces and is thus re-melted by other means.
A reverberatory furnace heats the furnace contents both by direct flame and by radiation from hot refractory linings and possibly additional heating elements. At its simplest, it is a steel box lined with alumina or other refractory brick having a flue at one end and a generally vertically lifting door at the other end closing a main entrance for the furnace through which a metal charge is normally introduced. Conventional oil or gas burners are usually placed on either side of the furnace to heat the brick and to melt the metal. The resulting molten metal is then poured into a casting machine to produce ingot. A static furnace may be tapped at the bottom by simply removing a ceramic-covered plug which then allows the molten aluminum to flow into a launder and then on to a casting machine itself. A convective furnace is similar but employs hot gases to heat the metal charge. Again, there is normally a main furnace entrance closed by a raisable door.
Aluminum scrap to be melted in such furnaces may be coated with organic materials (e.g. plastic coating layers) or may be uncoated. Coated scrap has to be melted in a side-well of the furnace where it is mixed with a flux that enables contaminants (dross) to be skimmed off before the molten metal enters the main chamber of the furnace. Uncoated (clean) scrap may be added directly to the main chamber of the furnace, but it is often wet or contaminated with water (e.g. water contained within internal shrinkage cavities formed in large castings). If water-contaminated scrap is added directly to the furnace, there is a risk of explosion, so the contaminating water has to be driven off before the scrap is introduced into the furnace. This may be done by positioning the scrap close to an entrance of the furnace and opening the furnace door to allow the furnace heat to pre-heat the metal charge for a period of time. Alternatively, the heating may be carried out in a drying chamber positioned at the furnace side well, again using heat from the furnace. The drying chamber typically has two doors, one at each opposite end. The first door is opened to load the scrap and the second door is opened after the scrap has been dried and the solid metal charge is pushed into the side well from which it migrates to the main chamber of the furnace. Unfortunately, both methods cause considerable heat loss from the furnace since a door for the main furnace entrance or the entrance to the side well is kept open to the atmosphere for a significant period of time. Even when a drying chamber is used, both doors are typically held open while the metal charge is transferred to the side well and hot air and radiant heat may pass freely through the chamber to the outside.
U.S. Pat. No. 2,757,925 issued to C. E. Carr on Aug. 7, 1956 shows the use of a charging car (container) having an open top. The car has a hinged wall at the front that swings down when the metal charge is pushed into the furnace by a ram. The wall provides a bridge that extends the reach of the container thereby allowing the solid charge to pass fully through the entrance in the front wall of the furnace. However, as the charge is being introduced into the furnace, there is a clear path from the furnace interior to the external atmosphere that allows the escape of heat. There is no suggestion that the car would be used to pre-heat the metal charge to remove contaminating water, and the design would not facilitate this because the open top of the car would allow heat to escape from the charge to the atmosphere (thereby making heating inefficient) and because the hinged wall is kept upright until the charge is pushed into the furnace (thereby blocking heat from the furnace from contacting the charge within the car).
U.S. Pat. No. 4,578,111 issued to Gillespie, John R., et al., on Mar. 25, 1986 discloses a melting furnace having a sweat chamber the floor of which forms a sweat hearth on which ingots or sows are placed. Combustion gases from the main furnace are used to pre-heat the ingots or sows, and then burners in the sweat chamber operate to melt the ingots or sows. However, the sweat chamber is a permanent part of the furnace and loading of the chamber with metal charge takes place through a slidable door that can release heated gases to the exterior as loading takes place.
It would be desirable to reduce or prevent such heat loss while facilitating the loading of scrap material into the furnace.